Ornamental grasses, valued for their texture, movement, and year-round interest, are a popular feature in many landscapes. These plants, which grow in dense, upright clumps, are generally low-maintenance once established. However, for those varieties that are perennial and form expanding clumps, division becomes necessary. This process involves separating the large plant into smaller sections to ensure the continued health and vigor of the stand. Understanding the correct timing for this maintenance is crucial for the plant’s survival and subsequent robust growth.
Why Ornamental Grasses Need Division
Clumping ornamental grasses naturally expand outward from their center over time, leading to overcrowding within the root system. As the plant matures, the older, inner sections of the root mass often become woody and cease producing new, healthy shoots. This internal decline significantly reduces the plant’s ability to absorb water and nutrients efficiently from the surrounding soil. Consequently, the grass may exhibit diminished flowering, thinner foliage, and an overall lack of vigor. Dividing the clump removes this unproductive, aging center and gives the younger, outer sections space and resources to thrive, effectively rejuvenating the entire plant.
Seasonal Timing for Warm-Season and Cool-Season Varieties
The optimum time to divide an ornamental grass is determined by its specific growth cycle, which falls into two main categories: warm-season and cool-season varieties. Dividing the plant when it is dormant or just beginning its active growth cycle minimizes the physiological shock to the root system. This timing allows the newly separated sections to establish themselves before the stress of peak growth or extreme weather arrives.
Warm-season grasses, which include popular varieties like Japanese Silver Grass (Miscanthus) and Fountain Grass (Pennisetum), begin their active growth later when soil temperatures rise. These grasses should be divided in late spring or early summer, just as the new green shoots are starting to emerge from the base. Dividing them too early, when the soil is still cold, risks exposing their sensitive roots to harsh conditions before they can recover.
Cool-season grasses, such as Blue Fescue (Festuca) and Feather Reed Grass (Calamagrostis), put on their most vigorous growth during the cooler temperatures of spring and fall. These varieties offer two windows for successful division. The preferred time is very early spring, immediately after the ground thaws but before the plant puts out significant new foliage. Alternatively, cool-season grasses can be successfully divided in early autumn, several weeks before the typical first hard frost in your region. This fall timing provides sufficient opportunity for the roots to establish themselves before the plant enters its winter dormancy. Dividing cool-season varieties during the heat of mid-summer, when they are under stress, should be strictly avoided.
Visual Indicators That Grasses Are Ready to Divide
While calendar timing is a helpful guide, the plant itself provides several visual cues indicating an immediate need for division. The most apparent sign is the development of a “dead center,” often described as a “donut hole” effect in the clump’s middle. This occurs when the oldest part of the plant has died out, leaving a ring of living growth around the perimeter.
A decline in the quality and quantity of the seasonal display also suggests the plant is root-bound and needs attention. This may manifest as fewer flower plumes or seed heads, or a noticeable reduction in the height and lushness of the foliage compared to previous years. For most ornamental grasses, this decline typically begins to appear every three to five years after the initial planting. If the root mass feels hard, woody, and impenetrable when trying to push a spade into the ground near the base of the plant, it is likely too dense and ready for separation.
Preparing and Dividing the Clump
Proper preparation of the plant is the first practical step in the division process, minimizing the physical trauma involved. Before digging, the foliage should be cut back severely, typically to within six to eight inches of the ground. This makes the clump easier to handle and reduces water loss through transpiration after replanting.
Dig around the perimeter of the clump, ensuring the spade is far enough out to capture the maximum amount of the root ball. Once the root ball is lifted, the hard, woody, and dead center section must be discarded, as it will not produce healthy new growth. Specialized tools like a sharp spade, a garden knife, or a handsaw are often necessary to cut through the dense, fibrous root mass.
Only the younger, more vigorous sections from the outer edges of the original clump should be retained for replanting. These viable sections should have a healthy amount of roots attached and can be planted immediately into prepared soil at their new location. After replanting, thorough watering is necessary to settle the soil around the new roots and help the divisions recover quickly from the shock of separation.